Reverse osmosis is utilized in membrane separation process whereby a feed stock containing a solute, which has molecular or colloidal dimensions which are significantly greater than the molecular dimensions of its solvent, is depleted of the solute by being contacted with the membrane at such pressure that the solvent permeates the membrane and the solute is retained. This results in a permeate fraction which is solute-depleted and a retentate fraction which is solute-enriched. In ultrafiltration, nanofiltration and microfiltration, pressure in excess of the osmotic pressure can be used to force the solvent through the membrane. Drinking water production, the production of milk protein concentrate for cheese production, and enzyme recovery are examples of ultrafiltration processes based on reverse osmosis.
Poly(phenylene ether)s are a class of plastics having excellent water resistance, thermal resistance, and dimensional stability. They retain their mechanical strength in hot, and/or wet environments. Therefore they can be used for the fabrication of porous asymmetric membranes useful in various separation processes, including reverse osmosis. For example, poly(phenylene ether)s can be used in processes that require repeated cleaning with hot water or steam sterilization. Nonetheless, there remains a need for a porous asymmetric membrane having improved filtration properties, including materials that will improve selectivity without adversely affecting permeation flux.
Processes for making hollow fiber membranes rely on using relatively large amounts of polymer additives such as polyvinylpyrrolidone (PVP) in the dope solution, which contains the membrane-forming polymer, in order to obtain the desired pore size, pore distribution, and contact angle for the selective per surface of the hollow fiber. The polymer additive must diffuse to the inner surface of the hollow fiber during coagulation of the membrane-forming polymer. Significant amounts of the polymer additive can remain trapped in the interior of the hollow fiber annular section and have little effect on pore formation on the inner selective layer. Also, the polymer additive can be incompatible with the membrane-forming polymer or insoluble in the dope solution.
A challenge is to develop a method for making porous asymmetric membranes in which the polymer additive does not cause incompatibility or solubility problems with the membrane-forming dope solution, and which efficiently delivers the polymer additive to the selective surface of the porous asymmetric membrane so the membrane properties for optimal membrane properties and performance.